Cytochalasin D, colchicine or vinblastine effectively inhibited both histamine release and 45Ca uptake induced by compound 48/80 in rat mast cells. The inhibitory effects of cytochalasin D or colchicine on histamine release were exerted more remarkably when permeabilized mast cells were stimulated with either Ca2+ or inositol-1,4,5-trisphosphate (IP3). Since colchicine, vinblastine or cytochalasin D were not effective in inhibiting IP3 formation, it was assumed that microtubules or microfilaments may not participate in the initial stages of the membrane events leading to histamine release. By contrast, in Ca2+ release from the intracellular Ca store both colchicine and vinblastine (but not cytochalasin D) were effective in inhibiting Ca2+ mobilization, indicating that microtubules, rather than microfilaments, are intimately related to Ca2+ release from the endoplasmic reticulum (ER). By means of a fluorescence microscope, it was revealed that colchicine decreased the fluorescence intensity of FITC-labeled anti-tubulin antibody in the mast cells, while the amount of tubulin polymer in mast cells increased after exposure to compound 48/80. The findings indicate that colchicine simply suppressed polymerization of tubulin, while the rearrangement of microtubules so as to increase the polymerization took place after exposure to compound 48/80. Using an electron microscope in combination with potassium antimonate technique, Ca-antimonate dots were clearly observed in a cluster on the surface of the ER and a distinct connection between the ER and microtubules was also observed. It was concluded that microtubules play an important role in the processes leading to Ca2+ release from the intracellular Ca store and in subsequent histamine release.